Due to the plurality of pairs of wires in a telephone cable from a central office (CO) or from a transceiver unit within the CO to connected subscribers or subscriber devices, respectively, crosstalk occurs despite a direct current insulation of the pairs of wires, both near-end crosstalk (NEXT) and also far-end crosstalk (FEXT). The same results from capacitive or inductive couplings. By twisting the individual pairs of wires of a cable, this crosstalk is minimized so far that it may be neglected in the voice band. In data transmission technologies, like e.g. DSL technology, however, apart from the voice band further frequency ranges are used, in which this crosstalk has a very strong effect on the possible range and transmission speed.
Near-end crosstalk means, that a transmitter on the “near side” of a receive device, i.e. both receive device and also transmitter are part of different subscriber terminal devices or both receive device and also transmitter are part of the central office, couples transmit signal portions into the receive branch of the receive device.
In contrast to that, far-end crosstalk means, that, in addition to the receive signal determined for the same, sent from a far-end first transmitter, i.e. first transmitter and receiver are on different sides of subscriber and central office side, a receiver receives portions of a second transmit signal sent from a far-end second transmitter, i.e. also second transmitter and receiver are on different sides.
In an xDSL network, wherein “x” stands for different implementations of DSL technology, the subscriber devices (CPE=customer premises equipment) are typically provided in a different distance from the central office (CO). A so-called near-far problem results from this. If all subscriber devices of the network transmitted with the same transmit power spectral density (TX-PSD, transmit power spectral density), the CPEs closer to the CO would respectively couple a high amount of crosstalk into the lines of the CPE further away from the CO and thus cause differently strong interference power spectral densities of the individual subscribers.
It is sometimes the case, that in an xDSL network ADSL lines (ADSL=asymmetric digital subscriber line) are provided together with VDSL lines (VDSL=very high speed digital subscriber line) in cable bundles. Due to higher data rates of VDSL systems, VDSL central offices are generally installed closer to the corresponding connected VDSL subscribers than comparable ADSL central offices with regard to their connected ADSL subscribers. The lines respectively starting from the central offices are combined in cable bundles and led in parallel at the end of the subscriber side for a few hundred meters, so that, for example, newly installed VDSL systems generate interferences by cross-talk on existing ADSL lines.
For this reason, within the scope of VDSL standardization (VDSL=very high-speed digital subscriber line)(see ITU-Standards G.993.1, G.993.2, G.997, ANSI-Standard T1.424-2004, ETSI-Standard TS101-270), a so-called Downstream Power Back-Off (DPBO) is defined. This is a method for protecting existing ADSL lines. It is the aim of the DPBO to form a transmit power spectral density (TX-PSD) at the VDSL central office side, such that the interference power spectral density caused by a VDSL wire pair on an ADSL subscriber side by far-end cross-talk comprises the same value as the FEXT-PSD generated at the same ADSL subscriber by a neighboring ADSL wire pair.
The indicated standards define a method which approximates the request for a respectively equal interference power density of the wire pairs at the CO by realizing the respectively equal receive power spectral density (RX-PSD). By this approximation, subscribers on shorter lines cause a lower FEXT-PSD than subscribers on longer lines. This disadvantage may be avoided by a correction value. The correction value depends on an electrical length or attenuation (EL, in dB), respectively, of the ADSL lines on a line length according to the distance between the ADSL central office and the VDSL central office, and on the electrical length or attenuation (CL, in dB), respectively, of the VDSL lines between the VDSL central office and the VDSL subscribers. Hence, EL corresponds to an information indicating a length of a part of an ADSL cable, the ADSL cable connecting an ADSL transceiver unit and an ADSL subscriber device, wherein the part of the ADSL cable extends from the ADSL transceiver unit to a point of the ADSL cable from where a VDSL cable via which a VDSL transceiver unit is connected to a VDSL subscriber device, joins the ADSL cable to extend to the VDSL subscriber device. Thereby the ADSL- and VDSL transceiver units can, for example, reside in ADSL- and VDSL central offices, respectively. To the TX-PSD in dBm/Hz determined according to the standardized method, a correction value
                              a          corr                =                              10            ·                                          log                10                            ⁡                              (                                  1                  +                                      EL                    CL                                                  )                                              ⁢          dB                                    (        1        )            is added. The line attenuations or electrical lengths EL and CL, respectively, may alternatively also be indicated in meters, as the ratio EL/CL does not depend on that.
The configuration parameter EL, also referred to as DPBOESEL in the aforementioned standards, defines the assumed electrical length of cables (E-side cables) connecting exchange based DSL services to a remote flexibility point (cabinet), that hosts the xTU-C (xDSL Transmission Unit-Central) that is subject to spectrally shaped downstream power back-off depending on this length DPBOESEL. For this parameter DPBOESEL the electrical length is defined as the loss (dB) of an equivalent length of hypothetical cable at a reference frequency defined by the network operator or in spectrum management regulations. According to the aforementioned standards, DPBOESEL shall be coded as an unsigned integer representing an electrical length from 0 dB to 255.5 dB in steps of 0.5 dB. All values in the range are valid. If DPBOESEL is set to zero, the DPBO in this section shall be disabled.